406 Life and Tmmortatty. 
ancient streams. But if the beds contain the remains of 
mollusks, minute crustaceans or fish, such as are found 
to-day in lakes, then we conclude that they are lacustrine, 
and were deposited beneath the waters of former lakes. 
And, lastly, if the remains of animals such as now people 
the oceans are to be met with in the beds, then we know 
that they are marine in origin, and that they are fragments 
of an old sea-bottom. On the whole, the conditions under 
which a bed was deposited, whether in a shallow sea, in the 
immediate vicinity of a coast-line, or in deep water, can 
often be determined with considerable accuracy from the 
nature of the relics of the organisms which they contain. 
But we have thus far been dealing with the remains of 
aquatic animals. When, however, we consider the remains of 
aerial and terrestrial animals, or of plants, the determination 
of the conditions of deposition is not made out with such 
an absolute certainty. Remains of land-animals would, of 
course, occur in sub-aerial deposits, that is, in beds, like 
blown sand, accumulated upon the land, but the most of 
such remains of such animals are found in deposits which 
have been laid down in water, and hence their present 
position is due to the fact that their former owners were 
either drowned in rivers or lakes, or borne out to sea by 
water-channels. Animals possessed of the power of flight 
might also similarly find their way into aqueous deposits, 
but, when it is remembered that many birds and mammals 
habitually spent a great part of their time in the water, it 
is not to be wondered at that they should present themselves 
as fossils in sedimentary rocks. Even plants, such as have 
undoubtedly grown upon land, do not prove that the bed 
in which they are found was formed on land, for many of 
their remains are extraneous to the bed in which they now 
occur, having reached their present site by falling into lakes 
or rivers, or by being carried out to sea by floods or gales of 
winds. Still, there are many cases which obviously show 
that plants have grown on the very spot where we now find 
